Screen printing is older than the electronic industry, and was initially developed in the first decade of this century out of the even older art of printing with stencils. In a modern screen printing process, a stainless steel screen is positioned on a ceramic or glass substrate, the screen having portions masked such that unmasked areas represent the areas to be printed. A printing ink containing a metal powder is then placed on the screen, and a squeegee is drawn over the screen to force the ink through the unmasked areas of the screen. The screen is then withdrawn, and the ink coated substrate is fired, leaving a pattern of metal conductors on the substrate.
The use of gold for thick film circuit fabrication has resulted in a significant advance in microelectronics. Gold is the most malleable and ductile of all metals, properties that assist in drawing fine wire and in wire bonding. Furthermore, gold does not directly combine with oxygen under any conditions. The absence of an oxygen reaction allows air firing of gold at temperatures up to its melting point. However a well known problem with the use of gold for printing inks for thick film conductors is that it is difficult to obtain adhesion between the gold and underlying substrate. In particular, while the lack of an oxide layer on the surface of gold is advantageous when forming a bond with a metal, such as in wire bonding, it is not advantageous for adhesive bonds with metal oxides such as glasses or ceramics.
Adhesion may be defined as the binding force exerted by molecules of unlike substances when brought into contact. Molecular level adhesion is a surface phenomenon, because only the surface molecules of the two bonded materials interact. The strongest molecular-level bond is the shared electron bond in which adjacent atoms share orbital electrons. For a shared electron bond to become effective, the atoms must come so close to each other that their electron shells overlap. Metals and metal alloys typically form shared electron bonds between atoms.
Electrostatic bonding is responsible for a different kind of adhesion mechanism. An electrostatic bond is based on electrostatic forces between molecules having electric poles, i.e., ions or dipoles. The molecules, when in close contact, will try to orient themselves so that the poles of opposite polarity attract one another. Such orientation is possible only if the molecules can move within their crystal structure. Electrostatic bonding, therefore, typically requires heating of materials to their softening points.
Oxides are electrically polarized, because of the shift of electrons during formation of the oxide molecule. Glasses, ceramics and other oxygen containing substrates upon which gold is deposited adhere to other substances through electrostatic bonds. Because gold has no affinity for oxygen, gold forms only shared electron bonds. A direct molecular bond of gold to substrate is therefore impossible, because the bonding mechanisms of the two materials are incompatible. Depositing gold directly on glass or ceramic results in a film without adhesion. This problem is overcome in thin film technology by the vacuum deposition of intermediate materials, such as chromium, on the glass before the gold is deposited.
The first generation of thick film gold inks obtained adhesion of gold to the substrate by the addition of glass frits to the gold powder. When a printing ink containing a glass frit is used, the glass melts during firing, flows between the gold particles under the influence of gravity and capillary action to the interface between the printed gold film and the substrate, and subsequently solidifies upon cooling. The glass wets the substrate and surrounds the gold particles, mechanically locking the gold film to the substrate. Particle size distribution of the gold powder significantly affects adhesive strength, because the gold film must be porous for mechanical interlocking by glass. High-density gold films therefore cannot be used in the frit bonding system.
Problems with frit bonded gold inks include low adhesive strength due to the low tensile strength of glass, the formation of glass coatings on the gold film that interfere with wire bonding, high sensitivity to firing parameters, and adhesion that varies with print thickness. In the past, in many circumstances, the top gold conductor of a multi-layer structure was overprinted with a fritless gold conductor, in order to cover any glass film with a clean gold bonding surface to obtain improved wire bonding characteristics.
About 15 years ago, a reactive bonding system was introduced, as exemplified by U.S. Pat. Nos. 3,799,890 and 3,799,891. The reactive bonding system is based upon the use of a copper oxide and/or cadmium oxide additive in powder form, and upon certain critical firing parameters. A disadvantage of the reactive bonding technique is that the fired gold film is coated with a relatively thick layer of copper oxide, making wire bonding extremely difficult. Another disadvantage is the comparatively high firing temperature necessary for bonding by the reactive bonding technique.
In general, the best prior approach to thick film fabrication has been to combine glass frits and reactive bonding agents as powders in the gold ink. A glass frit is added to carry the reactive copper oxide to the interface of the gold and substrate, and to serve as a filler for the gaps between the relatively rough gold film and the relatively flat substrate surface. The glass also increases the area of mechanical contact with the underside of the gold conductors, allowing the use of lower concentrations of bonding additives in the ink, resulting in less gold film surface contamination. Nevertheless, mixed-bonded inks still have the problems of surface contamination by glass and reactive bonding agents. These inks must also possess some porosity to allow the flow of molten glass to the gold/substrate interface, resulting in a softness undesirable in wire bonding. Even after the introduction of mixed bonded gold inks, many thick film fabricators continued to use an overprint step using a fluxless gold ink for the top conductor of multi-layer structures, to achieve maximum wire bondability.